Chapter 1 What’s New for FortiOS 5.2.1 : Logging and Reporting : Sniffer Traffic Log
  
Sniffer Traffic Log
Forward traffic from a FortiGate unit that is in one-arm sniffer mode can now be logged on that FortiGate unit. To log this traffic, the appropriate logging option must be selected for the sniffer interface.
Logging information can be viewed in the new sniffer log, which can be found by going to Log & Report > Forward Traffic > Sniffer Traffic.